The subject matter disclosed herein is generally directed toward systems and methods for obtaining knowledge of spacecraft attitude without using a gyroscope.
Due to limited resources or gyro failure, spacecraft attitude control systems (ACS) can face drastic challenges in mission accomplishment. Various ACS designs have been proposed to address satellite attitude control without a gyroscope. The critical life-saving maneuver when satellite attitude is lost in space is the so-called sun acquisition maneuver, in which the spacecraft is rotated about an axis normal to the sun line so that a solar panel sees the sun periodically and the rate of rotation about the sun line is regulated.
Spacecraft, such as an Earth-orbiting satellite, can become disoriented during flight. For spacecraft that use solar cells to generate power, in some cases the spacecraft disorientation can affect the proper spacecraft antenna pointing and the output of the spacecraft solar cells, resulting in reduced electrical power to the spacecraft. If the electrical power provided by the solar cells is low, it is advantageous to re-orient the spacecraft and/or solar panels so that the solar cells are perpendicular or nearly perpendicular to the Sun, along a sun line between the Sun and the spacecraft solar panels.
When the spacecraft solar cells are properly oriented relative to the Sun, it can be said that the spacecraft has acquired a proper orientation to the Sun. Information is needed to re-orient a disoriented spacecraft in three-dimensional space. Previously, a gyroscope and/or a star tracker (comprising, e.g., imaging hardware and a high-speed computer processor) were used to provide the needed information. However, both gyroscopes and star trackers are expensive and prone to failure.
A sun sensor senses the direction to the Sun and can be as simple as some solar cells and shades or as complex as a steerable telescope, depending on mission requirements. However, a sun sensor cannot detect a change in spacecraft attitude when the spacecraft rotates about the sun line, and can at most provide two-axis rate information. Alternatively, an earth sensor could be used together with a sun sensor when three-axis attitude and/or rate information is needed. One problem, however, is that an earth sensor cannot be used when the spacecraft is in a transfer orbit or pointing away from the Earth.
There is a need for systems and methods to provide information needed to re-orient a disoriented spacecraft without requiring a gyroscope, resulting in a more reliable and less costly spacecraft.